This invention relates generally to device and method for decoking delayed petroleum coke vessels, and more particularly to a rotary joint used to drive a drill stem and cutting tools, as well as an improved-reliability seal assembly used in the rotary joint. This invention even more particularly relates to a way to reduce operational downtime of a decoking tool by providing enhanced sealing of the rotary joint.
In a delayed coker operation of a petroleum refinery, residual feedstocks are first heated in a furnace and then moved to a vessel where the feed is allowed to coke by operating at a temperature sufficient to drive off the remaining volatile materials. After such heating the residue remaining in the vessel is essentially solidified petroleum coke. These vessels are currently as large as 32 feet in diameter and 150 feet in height.
The solid petroleum coke must be removed from the vessel in a decoking operation in order to prepare the coke for further hydrocarbon processing. Decoking is accomplished using high pressure water jets used to bore and cut the coke. The tool first bores a pilot hole through the coke bed and subsequently cuts the coke to the vessel wall, draining it out of the vessel through the pilot hole. This is accomplished by a tool or tools with arrays of vertical and horizontal nozzles to provide suitable jets for boring and cutting the coke bed. The tool or tools are rotated and reciprocated vertically through the vessel to accomplish the boring and cutting process.
Such tools are attached to a hollow drill stem through which the high pressure cutting water is conveyed to the rotating and reciprocating tool nozzles. The top end of the drill stem is attached to a rotary joint, which is used to bridge a non-rotating water supply line with the rotating drill stem and tool. The rotary joint is hoisted up and down within a tower above the coke vessel to impart the reciprocating motion to the assembly. The water supply line is in the form of a flexible hose that conveys the high pressure cutting water to the non-rotating portion of the rotary joint. The flexible nature of the water supply line allows it to accommodate the reciprocating motion of the assembly.
One important component within the rotary joint is the seal between the non-rotating and rotating portion of the rotary joint's water passage ways. Most current rotary joints in this service use lip type seals or packing, either of which tend to fail very quickly once a small leak develops. Such difficulties with seal leakage are exacerbated by the nature of the water being used in the decoking operation, as during the boring and cutting operations, the water is recycled, containing a quantity of suspended coke fines and related particulate. Changing a seal in a rotary joint can be time consuming and laborious process. Since delays in emptying the coke from the vessel result in loss of throughput in the overall refining process and concomitant operational impact on the refinery, having to shut down the process to change failed seals is commercially disadvantageous.
What is desired is a seal for the rotary joint that is not prone to failure. What is further desired is a seal that is easy and inexpensive to operate.